Glazing for the protection of ceramic surfaces is a common practice. Among other things, glazes are used on insulators for spark plugs, in which an appropriate glaze serves to protect the insulation ceramics against environmental effects and render its surface smooth and visually pleasing. In the past, glazes containing lead were used for this purpose; as a result of new developments, however, the use of lead is avoided for reasons of environmental protection.
Such lead-free glazes are, for example, known from European Patent Application No. 788 204, where the glazes are implemented on the basis of borosilicate glasses and contain, at most, only a small amount of lead. In addition, they may contain a fluoride in the form of sodium fluoride or aluminum fluoride.
The criteria important for good processing characteristics of a glaze include its melting point and the stability of the resulting glaze. The desired type of glaze is one with a sufficiently low melting point, and whose stability vis-à-vis crystallization processes and phase precipitations is as high as possible. European Patent Application No. 1 168 546 mentions glazes that have a zinc oxide content of 10 to 30 molar percent to stabilize the glaze. The fluoride content is limited to a maximum of 1 molar percent, but the addition of fluorides is actually undesirable.
In contrast to the related art, an object of the present invention is to provide a lead-free glaze for spark plugs, one that has a low melting point in addition to high stability in the melted state, adheres well to the glazed surface in the cured state, and ensures high mechanical strength of the glazed components of the spark plug.